Effects of carotid body chemoreceptor stimulation by dopamine (DA) on ventilation were studied by injecting it into the external carotid artery of rabbits. DA (3 micrograms--300 micrograms) induced dose-dependent ventilatory depression which was preceded by a transient increase in ventilatory rate in 65% of the experiments at higher doses. The DA-induced depression remained unaffected by prior administration of atropine, mecamylamine or phenoxybenzamine, while it was blocked or suppressed after haloperidol or reserpinization, indicating that the DA-induced ventilatory depression was due to its direct action on DA receptors in the carotid body. No response to adenosine triphosphate (ATP) was seen after the above treatments. The excitatory response to DA seemed to be mediated by a mechanism other than vasoconstriction, since it was not blocked by phenoxybenzamine and the pattern of the response was different from that of norepinephrine (NA).